Inkjet printing is a process in which a stream of ink, preferably in the form of droplets, is ejected at high speed from nozzles against a medium so as to create an image.
Media used for inkjet recording need to be dimensionally stable, absorptive of ink, capable of providing a fixed image and compatible with the imaging materials and hardware.
Most commercial photo quality inkjet media can be classified in one of two categories according to whether the principle component material forms a layer that is porous or non porous in nature. Inkjet media having a porous layer are typically formed of inorganic materials with a polymeric binder. When ink is applied to the medium it is absorbed into the porous layer by capillary action. The ink is absorbed very quickly but the open nature of the porous layer can contribute to the instability of printed images, particularly when the images are exposed to environmental gases such as ozone.
Inkjet media which have a non porous layer are typically formed of one or more polymeric layers that swell and absorb the applied ink. Due to the limitations of the swelling mechanism this type of media is slow to absorb the ink. However, once dry the printed images are often stable when subjected to light and ozone.
Alternatives to pure “porous” or “non porous” media are hybrids which take the merits of each pure medium. These hybrids have swellable porous layers. One such media is created from foamed polymer layers using a swellable hydrophilic polymer and blowing agents. This results in the formation of voids in the polymer layer which lead to improved absorption of the ink. Instead of the ink being held in pores which are located in-between particles, as in conventional porous media, the ink is located within the polymer. This results in improved image stability.
U.S. patent application Ser. No. 10/631,236 discloses an inkjet printing medium formed by a foamed polymeric layer. This medium may be created by the use of blowing agents.
It has been found that when blowing agents are added to the coating solutions prior to coating the surface of the final foamed polymeric inkjet media is quite rough. This is due to gas bubbles present in the melt, and therefore in the coating, acting as nucleation sites for further bubbles to form around. Coating quality can also be poor due to the pre-formed gas bubbles passing down the hopper slide. These gas bubbles cause lines, streaks and edge retraction.
It is an aim of the invention to provide a method of improving the surface characteristics and/or coating quality of a foamed polymeric material.